This invention relates to indicating meters having means for locking an indicating pointer or dial in a given position relative to an indicating scale of the meter, and more particularly relates to an indicating instrument pointer clamping mechanism that includes a bistable retainer for holding the clamping mechanism in either its clamping position or in a stored position, while providing a positive drive means to move the clamping member between said positions.
It is fairly conventional practice to provide certain types of portable indicating instruments with indicating pointer locking means whereby the indicating pointer of an instrument can be selectively locked in a given position relative to an indicating scale on the instrument. Typically such pointer clamping mechanisms are employed on instruments that are frequently used in circumstances such that a meter reading must be taken while the indicating scale of the meter is beyond the view of an operator. For example, a toxic gas instrument may be held in a confined space to monitor a suspected gas leak, thereby placing the indicating scale of the instrument in a position that is difficult or impossible for the operator to read. Alternatively, a clamp-on ammeter may be positioned around a conductor that is awkwardly placed such that an operator could not conveniently read the resultant current indication on the indicating scale of the instrument. In these cases, it is expedient for the operator to simply place the meter in the desired, somewhat inaccessible location to enable it to sense the monitored gas or current, as the case may be, then manually depress an indicating pointer clamping mechanism on the instrument to lock-in the meter reading so that the meter may be moved to a more accessible position in which the operator can readily read the recorded indication of the pointer relative to the indicating scale.
Examples of two generally known types of prior art indicating instrument pointer clamping mechanisms are shown respectively in U.S. Pat. No. 2,982,190--Fischer which issued on May 2, 1961 and U.S. Pat. No. 3,529,570--Englehard which issued on Sept. 22, 1970. As is somewhat typical of such indicating pointer clamping mechanisms, these patents illustrate the use of a generally U-shaped, pivotally mounted pointer clamping member that is operable to selectively engage an instrument indicating pointer and force it against the meter scale or a portion of the meter housing to lock the pointer in position in response to manual actuation by an operator. The generally U-shaped pointer clamping members of these mechanisms and related known prior art mechanisms are sufficiently wide to overlap the entire range of movement of the meter indicating pointer so that the clamping member can lock the pointer in position against the meter scale at any point thereon in which readings are capable of being taken. Another feature of these types of clamping mechanisms is that they employ resilient means to bias the clamp to its stored or non-clamping position. However, these mechanisms and related known prior art mechanisms do not provide for bistable latching of the pointer clamping member in both its pointer-clamping position and its stored position. Another shortcoming of such known commercially available systems is that they fail to provide for positive manual actuation of the pointer clamping member in both clamping and releasing positions thereof.
In order to assure free and accurate movement of an instrument indicating pointer relative to a clamping member associated therewith, while at the same time assuring a positive locking action of the clamping member relative to the indicating pointer when the clamping member is actuated, it is desirable to assure both firm engagement and positive disengagement between an indicating pointer and a clamping member when such modes of operation are desired. Specifically, it is most desirable to avoid any dragging relationship between the clamping member and the instrument indicating pointer while an instrument reading is being taken, prior to intentional movement of the clamping member to its clamping position.